Mass spectrometers work by ionizing molecules and then sorting and identifying the molecules based on their mass-to-charge (m/z) ratios. Several different types of ion sources are available for mass spectrometry systems. Each different ion source has particular advantages and disadvantages for different types of molecules to be analyzed.
Much of the advancement in liquid chromatography (LC/MS) over the last ten years has been in the development of ion sources. The introduction of techniques that are performed at atmospheric pressure have been of particular interest. These techniques do not require the use of complex pumps and pumping techniques to create a vacuum. Common techniques include and are not limited to electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI). Other more recent techniques include matrix assisted laser desorption ionization (MALDI) and atmospheric matrix assisted laser desorption ionization (AP-MALDI).
ESI is the oldest and most studied of the above-mentioned techniques. Electrospray ionization works by a technique that relies in part on chemistry of the molecules to generate analyte ions in solution before the analyte reaches the mass spectrometer. The liquid eluent is sprayed into a chamber at atmospheric pressure. The analyte ions are then spatially and electrostatically separated from neutral molecules. More recently, there has been a trend toward developing ESI ion sources that provide increased sensitivity, mass accuracy and signal stability.
Currently, the drying gas used in an ambient pressure ESI spray chamber is used for desolvating electrospray droplets. The drying gas then typically re-circulates in the chamber until it either exits the chamber via an opening or an exhaust port. Present systems suffer from the limitation of recirculation into the chamber, contamination by circulating species or sample residue. In particular, these contaminants can also impact signal strength which effects the quality of the final spectra obtained. For instance, signal strength can be reduced through poor ionization, poor efficiency in gathering the ions in the mass spectrometer and many other ways. In addition, a significant problem concerns the ability to maintain overall signal stabilities. Current methods and instrument designs provide a drying gas that enters the ionization chamber with a high velocity. The high gas velocity in many cases is needed to obtain the desired level of drying. However, this also negatively impacts spectra, spectra stability and overall instrument sensitivity. These and other problems have been overcome by the present invention.